The degree to which a wireless receiver can suppress interference affects several variables such as transmission power requirements and link utilization efficiencies in both the uplink and downlink directions of wireless communication systems. Better interference cancellation enables data transmission at lower power levels and/or at higher data rates than would otherwise be possible. The particulars of interference cancellation vary as a function of many variables, such as the communication signal types and protocols involved, details of the transmitting and receiving equipment, etc. However, providing good interference cancellation performance generally requires significant signal processing resources, because of the need to characterize and suppress received signal interference in real time.
For example, the well known generalized Rake (G-Rake) receiver uses a plurality of fingers to suppress interference and improve demodulation. The interference might result from other symbols of interest (self-interference), symbols intended for other users in the same cell (own-cell interference) or symbols intended for other users in other cells (other-cell interference). The fingers capture information about the interference environment and this information is used to suppress the interference. Each finger of the G-Rake receiver includes a correlator placed at a particular processing delay, also referred to interchangeably herein as a finger delay. Impairment cross-correlations between the fingers can be represented as an impairment covariance matrix Ru. The impairment covariance matrix can in turn be used to generate combining weights used by the G-Rake receiver to combine de-spread data values. The G-Rake receiver uses the impairment covariance matrix Ru to whiten colored interference in the received signal(s) of interest by computing a combining weight vector, w, as given by:w=Ru−1h  (1)where h is the net channel response vector. Each element of h represents the overall propagation channel response between a signal transmitter and a receiver finger, including the radio channel as well as the transmitter and receiver pulse-shaping filters.
The overall impairment covariance matrix Ru used in determining the G-Rake combining weights is typically given by:
                              R          u                =                                                            E                c                            ⁢                              R                I                                      +                                          N                0                            ⁢                              R                n                                      +                                          ∑                                  j                  =                  1                                J                            ⁢                                                          ⁢                                                E                  c                  j                                ⁢                                  R                  O                  j                                                              ⁢                                          ⁢                                          =                                                    N                0                            ⁢                              R                n                                      +                                          ∑                                  j                  =                  0                                J                            ⁢                                                          ⁢                                                E                  c                  j                                ⁢                                  R                  O                  j                                                      -                          hh              H                                                          (        2        )            where Ec is the average energy transmitted per chip of an own-cell base station, N0 is a one-sided power spectral density of white noise, Ecj is the average energy transmitted per chip of a jth other-cell base station, R1 is an own-cell interference covariance matrix, Rn is a covariance matrix representing white noise passed through a pulse shaping filter and ROj is a jth other-cell interference covariance matrix. Net channel coefficients are represented in equation (2) by a vector h which includes the effect of transmit/receive filters in addition to the radio channel.
For parametric G-Rake interference cancellation techniques, the computation of R1 is the major contributor to overall algorithm complexity for receivers that do not model other-cell interference (i.e. ROj is modeled as white noise). As such, the complexity of a parametric G-Rake receiver can be reduced by improving the efficiency of the R1 computation, freeing computational resources for other receiver tasks. The second formulation of equation (2) emphasizes that own-cell interference can be computed in a similar fashion to other-cell interference provided that a benign signal term is subtracted (i.e. R1=RO0−hhH). Thus, even further computational reductions can be realized by improving the efficiency of the R1 computation and the ROj computation.
Some conventional techniques for calculating R1 and/or ROj involve reformulating the calculations for the elements of R1 from:
                                                        R              I                        ⁡                          (                                                d                  1                                ,                                  d                  2                                            )                                =                                    ∑                              I                =                0                                            L                -                1                                      ⁢                                                  ⁢                                          ∑                                  q                  =                  0                                                  L                  -                  1                                            ⁢                                                          ⁢                                                g                  I                                ⁢                                  g                  q                  *                                ⁢                                                      ∑                                                                  m                        =                                                  -                          ∞                                                                    ,                                              m                        ≠                        0                                                                                    m                      =                      ∞                                                        ⁢                                                                          ⁢                                                                                    R                        p                                            ⁡                                              (                                                                              d                            1                                                    -                                                      mT                            c                                                    -                                                      τ                            I                                                                          )                                                              ⁢                                                                  R                        p                        *                                            ⁡                                              (                                                                              d                            2                                                    -                                                      mT                            c                                                    -                                                      τ                            q                                                                          )                                                                                                                                ⁢                                  ⁢        to                            (        3        )                                                      R            I                    ⁡                      (                                          d                1                            ,                              d                2                                      )                          =                                            ∑                              m                =                                  -                  ∞                                            ∞                        ⁢                                                  ⁢                                          h                ⁡                                  (                                                            d                      1                                        -                                          mT                      c                                                        )                                            ⁢                                                (                                      h                    ⁡                                          (                                                                        d                          2                                                -                                                  mT                          c                                                                    )                                                        )                                *                                              -                                    h              ⁡                              (                                  d                  1                                )                                      ⁢                                          (                                  h                  ⁡                                      (                                          d                      2                                        )                                                  )                            *                                                          (        4        )            Note that equation (4) requires net channel estimates at chip-spaced intervals. This implies that a grid-like finger placement enables the efficient computation of equation (4) provided the receiver has a sufficient number of fingers.
The grid of fingers available for interference suppression should be of sufficient extent (e.g. density and scope) to yield a reasonable approximation to the infinite summation in equation (4). Absent a sufficient number of fingers, performance degradation occurs. In addition, the grid of fingers used for interference suppression may be precluded from being reused for dual Rake/G-Rake operation. During dual Rake/G-Rake operation, the Rake receiver is used for control channel demodulation and the G-Rake receiver is used for traffic channel demodulation in both uplink and downlink directions. As a specific example, suppose the channel consists of two paths having delays of Tc and 3Tc/4 where Tc correspond to the chip sampling rate. Under these conditions, a chip spaced or half-chip spaced grid of fingers cannot be constructed for interference cancellation that includes these path delays, requiring additional fingers to enable dual Rake/G-Rake operation. In some cases, additional fingers may not be available. Furthermore, the channel estimates used in computing equation (4) tend to be noisy. As a result, the interference estimates yielded by equation (4) are likewise noisy. This causes performance degradation compared to a receiver with ideal channel estimates. The performance degradation can be significant for a highly dispersive channel.